Tropical cyclones form and intensify under a specific set of necessary conditions. These conditions must be met in order to a tropical cyclone to form; however, the presence of these conditions does not guarantee development.
Six Necessary Conditions for Development
- Warm ocean waters of at least 78°F (26°C)
- At least 5° latitude from the equator
- Low vertical wind shear
- Moisture in the mid-troposphere
- Unstable Conditions
- Pre-existing disturbance
1. Warm Ocean Waters
Tropical cyclones thrive off the heat content of the warm tropical and subtropical oceans. Warm ocean water evaporates from the surface and is taken in by the developing cyclone. As the water vapor rises, it eventually condenses into liquid water, forming clouds and releasing energy in the form of heat. Tropical cyclones use the released energy to gain organization and strength.
Thus the warmer the water, the more energy can be extracted during condensation, and the stronger the storm can grow. However, tropical cyclones require a large temperature difference between the ocean surface and upper-level air to be most efficient at turning the heat energy into mechanical energy (in the form of strong winds).
2. At least 5º latitude from the equator
Tropical cyclones are well known for their rotation, which is easily visible from a satellite loop of the storm. However, in order to acquire this spin they must obtain it from the rotation of the earth. At the equator the influence of earth's spin is zero and it increases moving toward the poles (this is known as Coriolis force). While there is nothing magical about 5º degrees latitude, there is just enough influence from the earth's spin to be favorable for tropical cyclone development.
3. Low Vertical Wind Shear
Vertical wind shear is the difference in wind speed and direction at two different heights in the atmosphere. An example of shear would be to have wind increasing rapidly with height. Tropical cyclones have trouble developing in high shear environments because it displaces the thunderstorms in the cyclone away from its center of circulation. The thunderstorms are the primary location for the cyclone to gain energy through condensation (thinking back to the first criteria). By moving the thunderstorms away from the center of circulation, the rotation weakens because it is not receiving a steady supply of energy.
4. Moisture in the mid-troposphere
Tropical cyclones require a moist mid-troposphere to help keep their clouds and thunderstorms in tact. Dry air introduced into the mid-levels will begin eating away at the cyclone's clouds. Furthermore, the dry air can become wrapped into the circulation of the cyclone, disrupting the thunderstorms and causing asymmetries in the structure of the cyclone.
5. Unstable Conditions
Tropical cyclones contain thunderstorms, which ingest moist air near the surface. The most effective way to bring the air near the surface up into the thunderstorms is if that air is unstable. Unstable air is typically very warm, moist air (such as over the tropical oceans) that when lifted will continue to rise on its own. The more unstable the air is, the more rapidly it will rise. Rapidly rising air helps to create vigorous thunderstorms which contribute to the overall tropical cyclone.
6. Pre-existing disturbance
A pre-existing disturbance can be considered a seedling that, if placed under the previous five conditions, may grown to be a tropical cyclone. While a disturbance can come in many different forms, the most typical in the Atlantic Basin are African Easterly Waves. These waves are generally a complex of thunderstorms that moves off the west coast of Africa. Provided the previous 5 conditions are in place over the Atlantic Ocean off the west coast of Africa, a disturbance moving over that area may develop.